1. Field of the Invention
The present invention relates generally to field disturbance sensors and more particularly relates to Doppler radar sensors adapted to sense the relative motion of life forms which are located behind a vehicle.
2. Description of the Prior Art
The problem of detecting people and animals in an area behind a vehicle is well established. This problem is particularly serious in the field of farm vehicles (e.g., tractors) and lawn mowing equipment. Presently, thousands of serious accidents occur each year resulting from the misuse of such equipment. These accidents are particularly tragic when they involve small children that have come up behind the vehicle while playing. A large percentage of the resulting injuries are inflicted when, for example, a tractor is backing up and inadvertently strikes an unseen victim. These injuries, including impact injuries and lacerations from contact with a spinning mower blade, are oftentimes severe.
Various forms of sensors have been employed in collision avoidance systems for vehicles. Among these systems are radio-based sensors (radar), ultrasonic sensors, laser-based radar and infrared (IR) sensors, each of which offers unique properties. Of these sensor technologies, Doppler radar sensors are particularly well suited for applications where it is desirable to detect human and/or animal motion.
Doppler radar sensors, which are well known in the art, are based on the principal of Doppler shift. In simple terms, energy waves (e.g., sound or electromagnetic) reflected off a moving target will exhibit a shift in frequency. The amount of frequency shift is directly proportional to the body's radial velocity relative to the source (either approaching or receding). This Doppler frequency shift f.sub.D obeys the relationship EQU f.sub.D =2v.sub.R f.sub.o /c
where:
f.sub.o =transmitted frequency PA1 c=velocity of light (3.times.10.sup.8 m/s) PA1 V.sub.R =relative velocity of body with respect to source (m/s)
Vehicles, such as lawn tractors, farm implements and the like, typically operate in an open outdoor environment which is subject to various sources of sensor interference. A typical environment may include fences, plastic trim or lawn furniture, dried wood, and/or concrete curbs which substantially interfere with the operation of conventional proximity sensors. The environment may also include thermal "hot spots," such as areas where solar heating occurs on the ground, which interferes with the operation of standard infrared sensors. Therefore, there remains a need for a protection sensor system for a vehicle which substantially overcomes the problems associated with devices known in the prior art.